Owners and users of motorcycles, all terrain vehicles, snowmobiles, commonly called powersports vehicles and other special purpose vehicles such as golf carts and powered wheel chairs are frequently modified from their original equipment manufactured configuration to add a variety of accessories that enhance the vehicle capability and the operator's experience. These added accessories may be roughly divided into three categories: low power up to 2 amp or 20 watts, medium power of 2 to 5 amps or 20 to 60 watts and high power of 6 amps or 80 watts and greater. Examples of low power accessories may include navigation systems such as global positioning systems, cell phones, entertainment systems, radar detectors, intra-vehicle rider to passenger communication systems, inter-vehicle short range communication systems, personal digital assistants and small computers. Medium power accessories may include on-road auxiliary lighting and single articles of heated clothing. High power devices may include high intensity off-road lighting, external battery chargers and multiple articles of heated clothing
Care must be exercised when adding accessories to vehicles to ensure that the electrical capacity of the vehicle system is not exceeded. Both the battery and charging system must be considered when evaluating electrical loads. Calculating the electrical load effect on the charging system is straight forward as it is related only to the output capacity of the charging device; almost always an alternator in most vehicles. However, the effect of added load on a battery is problematic in that battery output can be significantly affected by temperature and battery condition. For example the greatest battery load is under starting conditions at low temperatures. At low temperatures engine cranking loads are high due to increase lubricant viscosity and component friction. At the same time battery output is reduced by the low temperature. It is therefore worthwhile to disconnect unnecessary electrical loads during engine cranking.
Simply switching high power accessories off during cranking is one option; however it is unrealistic to expect a user to switch these devices off before initiating the starting process. The only way to ensure consistent disconnect is to automate the action.
In the simplest form the disconnection could be accomplished with a relay with normally closed contacts. Activating the relay by means of the start function would open the relay contacts and thus disconnecting the electrical load. This method has a major drawback. It allows the electrical load to discharge the battery from the time the vehicle ignition is turned on to a point when the start function is initiated.
Moreover, this on-off-on load sequence may have a negative effect on some accessories. A case in point is high intensity discharge (HID) lighting. HID lights are illuminated by an arc that is formed across contacts. Igniting this arc requires very high voltage and moderately high current. Igniting the arc when the ignition is turned on, then extinguishing it during starting only to reignite when the engine starts can be deleterious to the igniter and arc contacts.
Given the above, it is clear that it would be of significant advantage to prevent unnecessary accessory electrical loads from turning on until the engine was running at sufficient speed to ensure that the charging system is providing sufficient output to handle those loads. This control may be implemented is various ways: by time delay, by monitoring oil pressure or by monitoring engine speed. As will be shown, the only effective method is the monitoring of engine speed.
A time delay method is not reliable due to the variable time that is required to ensure that the engine is running. If the set time is too short the accessory may turn on while the engine is cranking. If the time is too long the operator may have to wait for the accessory to turn on before operating the vehicle. This could lead to dangerous situations such as a stall in traffic at night.
Controlling the accessory load with oil pressure is equally unreliable. If the threshold pressure is set too low the engine cranking speed may be sufficient to bring the oil pressure up to a point where the accessory may turn on. If set too high the accessory may turn off when the engine speed drops below the threshold such as at engine idle. This again could be extremely dangerous during nighttime operation.
Controlling the accessory load with engine speed or charging system output overcomes these drawbacks. Setting the threshold engine speed at a point between engine cranking and low idle ensures that the engine is running at sufficient speed in order that the charging system is generating enough output to support the added accessory load.